DSSS systems include GPS transmitter/receiver systems that perform time-of-arrival (T.O.A.) determinations between a DSSS transmitter (such as a GPS satellite) and a DSSS receiver (such as GPS user equipment). The present invention applies to all DSSS Systems, but will be described for convenience with request to a GPS system herein. Multipath problems within GPS systems are well known and take a variety of different forms including ground reflection, single reflection (diffraction), diffuse multipath, and mobility-induced errors. Each of these is discussed below:
(1) Ground reflection. A strong multipath signal between a GPS satellite and a GPS user equipment (for example 100% reflected power) can originate from ground or sea surface reflections, as shown in FIG. 1. If the angle E is a low elevation angle and/or the user altitude h is low, the multipath may be only slightly delayed from the main path between the GPS satellite and the user equipment. If the multipath delay is less than 1.5 chips, the multipath delay degrades the signal TOA estimation. For C/A-code, this condition occurs for:Delay =ΔR/c=2h sin E/c<1.5 μs.
Ground or sea surface reflections and the effects thereof are described in Spilker, “Overview of GPS Operation and Design,” American Institute of Aeronautics and Astronautics, Inc. Vol. 163 at page 53 (1994). A typical behavior of the ground reflection error after smoothing as the satellite travels across the sky is described in Brenner, “GPS Landing System Multipath Evaluation Techniques and Results,” at pages 1001-2.
(2) Single Reflector (or Diffractor). Another multipath problem occurs when a single reflector or diffractor creates a multipath signal between the user equipment and the GPS satellite. Such a multipath problem is illustrated in FIG. 2, where the main path signal (on top of FIG. 2) and the multiple path (on the bottom of FIG. 2) between a user equipment and GPS satellite are shown being created by the diffractor. Brenner, “GPS Landing System Multipath Evaluation Techniques and Results,” ION GPS-98, at pages 1000-1001 (Conference on September 15-18, 1998) also describes the resultant multipath-induced position error, which is reproduced by way of example in FIG. 3.
(3) Diffuse multipath. Another type of multipath problem occurs with uniformly scattered diffractors, as shown in FIG. 4. In the case of uniformly scattered diffractors, the power equals Pdiffr (0.2/dr2). The smoothed multipath error in the case of diffuse multipaths is described in Brenner at pages 1000-1001.
(4) Effect of mobility. Still another multipath problem that exists between GPS satellites and user equipment occurs as a result of mobility in the user equipment. According to Van Nee, “Multipath Effects on GPS Code Phase Measurements,” Navigation, Vol. 39, No. 2, Summer 1992 at pages 179-180, the motion of the user equipment causes differences in Carrier Doppler frequencies between reflections in the line of sight of stationary users versus mobile users. That is, the fading bandwidth is determined by the Doppler differences, which varies substantially for stationary users versus mobile users. Further, reduction of the multipath error variance requires an averaging time much greater than 1/fading bandwidth. For stationary users, maximum Doppler difference is 0.6 Hz (and most users even experience much lower values). Mobile users, however, experience much higher fading bandwidth, e.g., for v=15 m/s, Doppler differences can take values up to 180 Hz. As a result, smoothing techniques require long time constants (on the order of 100 seconds) for stationary users but can use much shorter time constants for mobile users.
Still further multipath signal generation can be caused by terrain (such as) urban canyons and by signal reception within buildings.
There are current receiver technologies that attempt to mitigate the multipath disorders. Current GPS transmitter/receiver systems attempt to perform time of arrival determinations on the GPS spread spectrum waveform by (1) correlating the received signal with a replica of the transmitted signal and then (2) finding the time location of the peak magnitude of the correlation. They either locate the peak directly or by curve-fitting an ideal correlation function (a triangular pulse) with the actual received signal correlation function. Three recent systems for receiver technology improve the TOA estimation accuracy of the GPS receiver in a multipath environment:
(a) Narrow correlator (Novatel™). The narrow correlator uses a correlator spacing of a fraction of a chip rather than chip-spaced correlators, as illustrated in FIG. 5. Using a fraction of a chip greatly reduces the magnitude of the maximum TOA error in a multipath environment. The error reduction by the narrow correlator is described by Van Dierendonck et al., “Theory and Performance of Narrow Correlator Spacing in a GPS Receiver,” Navigation, Vol. 39, No. 3, Fall 1992, at pages 265-283 and reproduced by way of summary in FIG. 6. In FIG. 6, the multipath error for a single diffractor, with narrow correlation is illustrated.
Others have extended narrow correlators to P(Y) code receivers, e.g., Karels, et al. “Extending Narrow-Correlator Technology to P(Y)-Code Receivers: Benefits and Issues,” ION GPS-94 Sep. 20-23, 1994, investigated using narrow-correlator techniques on P(Y)-code receivers. Karels et al opined that improvements in overall GPS receiver performance obtained in commercial GPS receivers over standard C/A-code receivers may be extended to P(Y)-code GPS receivers but only if the GPS space vehicle (SV) spectral output is permitted to increase. A table of Karels, et al., finding is reproduced as FIG. 8.
(b) Multipath estimating delay lock loop (MEDLL) (Novatel™). MEDLL makes multipath error correction by assuming that no more than two dominant multipath signals are present. It estimates the amplitude, delay, and phase of each multipath component using maximum likelihood criteria and then subtracts each estimated multipath correlated function from the measured correlation function. The remaining direct path correlation function has minimal multipath degradation, and it can be used for accurate TOA estimation. The technique is described by Towsend, et al., “Performance Evaluation of the Multipath Estimating Delay Lock Loop,” ION National Technical Meeting Jan. 18-20, 1995, and exhibits multipath error correction illustrated by way of summary in FIG. 7. In practicality, the MEDLL technique gives performance that's comparable to a p-code receiver.
(c) Leading edge curve fitting. A third system for improving TOA estimation accuracy of a GPS receiver in a multipath environment is the leading edge curve fitting technique first used by the present assignee for W-sensor applications and for small unit operations programs. The leading edge curve fitting technique matches the receive signal correlation with an ideal correlation function on the leading edge of the received signal correlation. This minimizes the impact of any delayed multipath signals when computing the TOA, because the multipath has its greatest influence on the trailing edge of the correlation.
The preferred embodiment of the present invention provides a more useful receiver technology for improving the TOA estimation accuracy by utilizing a QMFR technique in the GPS application. This technique reduces the influence of close-in multipath components by examining the complex correlation function of the received signal rather than just the magnitude of the correlation function. It uses curve fitting on the complex correlation signal to locate the correlation peaks due to the main (undelayed) path in the delayed multipath component, and then measures the phase angle between those peaks.